A number of studies have provided evidence that human fertility has declined over recent decades, raising alarm that a widespread reproductive toxicant exists in the environment. We carried out simulations to assess the bias due to recent trends in the availability of effective methods of birth control, including induced abortion, and showed that contradictory reports in the literature can potentially be explained by demography-induced biases that have different effects on different study designs, and that the study of trends in fertility is particularly susceptible to bias and misinterpretation. In another project, we assessed the effect of errors in estimating gestational age on survival-analysis-based assessments of risk of spontaneous abortion in relation to exposures during pregnancy. The results were reassuring in that effects are small, except in implausible scenarios. In other work we showed that the strong relationship between birthweight and perinatal mortality could be reproduced if a very few babies are afflicted by a factor that has both a marked effect on fetal growth and a large effect on mortality, thus confounding the relation between birth weight and mortality. The fact that one can fit the mortality curve for birth weight extremely well with a simple confounding model calls into question the common assumption that public health measures aimed at 'improving' birth weight will reduce perinatal mortality. It also raises the possibility that a rare factor that influences both fetal growth and perinatal mortality may be important to identify. In another project with a high school student intern, we developed calculations to estimate the daily amount of human chorionic gonadotropin (hCG) produced by the early human embryo in the few days following implantation. This was done by accounting for the first-order kinetics of hCG clearance to derive the new production needed to account for increasing urinary levels seen in women in the Early Pregnancy Study. We also modeled clearance rates following early pregnancy loss and studied the variation in clearance rates across women. We found that women with a high body mass index tend to have more rapid clearance of hCG.